256 WM. REES B. ROBERTSON 



a pair of proximal knobs or arms (my figs. 156d, 156e; '08, figs. 

 29b, 29f, fig. 30, no. 11, fig. 32, nos. 7 and 9), these investigators 

 have represented them as crossing each other in the manner of 

 Janssens's text figm^e XXI for crosses, or as Davis ('08) has rep- 

 resented at figures 179, 180 for the simple knobbed rings. In 

 some cases I admit that a free crossing resulting from a twisting 

 of the chromosomes, rings, etc., may have occurred, but I am 

 certain, especially in the case of the long chromosomes of Steno- 

 bothrus, that many of these so pictured crossings do not occur, 

 and that the chromosomes have been misrepresented in illus- 

 trations (e.g., Davis, '08, figs. 86, 87, 183-186; Meek, '11, figs. 10, 

 14-16, 18; '12, figs. 276-279; de Sinety, '01; Gerard, '09). 



I do not believe that the breaking and secondary fusion of 

 crossing filaments which Janssens has postulated can take 

 place after, or at the time of, stage 'h.^ If they take place at 

 all, it must be in the zygotene or pachytene stage, when the 

 pairing threads twist about each other more or less intimately 

 and for a time appear fused. That such breaking and second- 

 ary fusion as he postulates occurs, I am not quite ready to ad- 

 mit, in view of the evidence to the contrary given by the unequal 

 homologous chromosomes of Acridium and Tettigidea ('4s in 

 figs. 115, 120, 122, and I's in figs. 141-147) and by the V-rod 

 tetrad of Jamaicana (figs. 197-199). In Tettigidae I am cer- 

 tain (Studies II) that parasynapsis takes place. Now, if there 

 be such a fusion as Janssens says occurs in the early strepsitene 

 stage, we should except to have irregularities in the lengths of 

 the exconjugating chromosomes shown at the late metaphase and 

 anaphase of the first maturation division (4's in figs. 115, 120, 

 122 and I's and I's in figs. 141-147). There is, however, no 

 visible variation in the relative length of these separating chromo- 

 somes. This matter I have taken up also in Study III. 



Something similar may be said of the V in Jamaicana, which, 

 if we assume that parasynapsis occurs, we have every reason 

 to believe must be paired with two rods (nos. 14 and no. 16) dur- 

 ing such a stage as the pachytene or strepsitene. If fusions take 

 place of the type Janssens supposes, we should expect the occur- 

 rence of irregularities in the form of the exconjugating mem- 



